(a) Technical Field
The present invention relates to a bypass device for a fuel cell stack. More particularly, it relates to a temperature-sensitive bypass device for discharging condensed water from a fuel cell stack, which can easily discharge an excessive amount of condensed water introduced into the fuel cell stack during cold start-up or during initial operation, thus ensuring stable operation of the fuel cell stack.
(b) Background Art
First, the configuration of a fuel cell stack will be briefly described with reference to FIG. 10.
A membrane-electrode assembly (MEA) is positioned in the center of each unit cell of the fuel cell stack, and the MEA comprises a solid polymer electrolyte membrane 10, through which hydrogen ions (protons) are transported, and an electrode/catalyst layer such as a cathode (“air electrode”) 12 and an anode (“fuel electrode) 14, in which an electrochemical reaction between hydrogen and oxygen takes place, disposed on each side of the polymer electrolyte membrane 10.
Moreover, a gas diffusion layer (GDL) 16 and a gasket 18 are sequentially stacked on both sides of the MEA, where the cathode 12 and the anode 14 are located. A separator 20 including flow fields, through which reactant gases (such as hydrogen as a fuel and oxygen or air as an oxidant) are supplied and coolant passes, is located on the outsides of each GDL 16.
After several hundreds of unit cells are stacked, an end plate 30 for supporting and fixing the unit cells is connected to each end of the fuel cell stack. Further, a current collector for collecting electricity generated in the stack and supplying the electricity to the outside is mounted on the inside of each end plate 30.
An oxidation reaction of hydrogen occurs at the anode 14 of the stack to produce hydrogen ions (protons, H+) and electrons (e−) by a catalyst disposed in the electrode/catalyst layer. The hydrogen ions and electrons are transmitted to the cathode 12 through the electrolyte membrane 10 and the separator 20. At the cathode 12, water is produced by the electrochemical reaction between the hydrogen ions and electrons transmitted from the anode 14 and the oxygen-containing air. Electrical energy generated by the flow of electrons is supplied to a load that uses the electrical energy through the current collector of the end plate 30.
The above-described polymer electrolyte membrane fuel cell typically operates at a low temperature of 60 to 90° C., and thus efficient water management is necessary.
In particular, when an excessive amount of condensed water is introduced into the stack during abnormal operation, such as cold start-up or during low power operation for a long time, the condensed water interferes with the efficient supply of reactant gases. As a result, the performance and durability of a fuel cell system is reduced.
For example, the condensed water present in fuel (hydrogen and oxygen in air) supply lines or in a hydrogen recirculation line is introduced into the stack at the same time during cold start-up, which causes rapid deterioration in performance of a cell adjacent to an inlet of the stack.
Moreover, when the fuel cell system operates at low power operation for a long time, the water produced by the reaction is continuously circulated under conditions where the temperature of the stack is not very high, which causes deterioration in performance of the fuel cell.
As shown, for example, from the test results in FIGS. 8 and 9, the excessive amount of condensed water separated from the reactant gases (hydrogen and oxygen in air) and most of the condensed water was introduced into the cell adjacent to the inlet of the stack during cold start-up of the vehicle equipped with the fuel cell stack.
In an attempt to solve this problem caused by the condensed water, a water trap or gas/liquid separator has been provided at the front and rear ends of the stack to remove the condensed water. However, such water traps and gas/liquid separators only separate a small amount of water droplets mixed with the gaseous fuel, and they do not remove the excessive amount of condensed water introduced and accumulated in the inlet of the stack.
Accordingly, there remains a need in the art for an apparatus and method for discharging condensed water from a fuel cell stack.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.